This invention relates to a novel method and apparatus for offshore drilling operations. More specifically, this invention relates to a method and apparatus for conducting exploration drilling offshore, with a single derrick wherein primary and auxiliary exploration drilling operations may be performed simultaneously to shorten the critical path of primary drilling activity. In addition, this invention relates to a method and apparatus wherein a single derrick is operable to perform multiple drilling, development, and work over operations simultaneously.
In the past, substantial oil and gas reserves have been located beneath the Gulf of Mexico, the North Sea, the Beaufort Sea, the Far East regions of the world, the Middle East, West Africa, etc. In the initial stages of offshore exploration and/or development drilling, operations were conducted in relatively shallow water of a few feet to a hundred feet or so along the near shore regions and portions of the Gulf of Mexico. Over the years, the Gulf and other regions of the world have been extensively explored and known oil and gas reserves in shallow water have been identified and drilled. As the need for cost effective energy continues to increase throughout the world, additional reserves of oil and gas have been sought in water depths of three to five thousand feet or more on the continental shelf. As an example, one actively producing field currently exists off the coast of Louisiana in two thousand eight hundred feet of water and drilling operations off New Orleans are envisioned in the near future in approximately three thousand to seven thousand five hundred feet of water. Still further, blocks have been leased in fields of ten thousand feet and by the year 2000 it is anticipated that a desire will exist for drilling in twelve thousand feet of water or more.
Deep water exploration stems not only from an increasing need to locate new reserves, as a general proposition, but with the evolution of sophisticated three dimensional seismic imaging and an increased knowledge of the attributes of turbidities and deep water sands, it is now believed that substantial high production oil and gas reserves exist within the Gulf of Mexico and elsewhere in water depths of ten thousand feet or more.
Along the near shore regions and continental slope, oil reserves have been drilled and produced by utilizing fixed towers and mobile units such as jack-up platforms. Fixed towers or platforms are typically fabricated on shore and transported to a drilling site on a barge or self floating by utilizing buoyancy chambers within the tower legs. On station, the towers are erected and fixed to the seabed. A jack-up platform usually includes a barge or self-propelled deck which is used to float the rig to station. On site legs at the corners of the barge or self-propelled deck are jacked down into the seabed until the deck is elevated a suitable working distance above a statistical storm wave height. An example of a jack-up platform is disclosed in Richardson U.S. Pat. No. 3,412,981. A jack-up barge is depicted in U.S. Pat. No. 3,628,336 to Moore et al.
Once in position fixed towers, jack-up barges and platforms are utilized for drilling through a short riser in a manner not dramatically unlike land based operations. It will readily be appreciated that although fixed platforms and jack-up rigs are suitable in water depths of a few hundred feet or so, they are not at all useful for deep water applications.
In deeper water, a jack-up tower has been envisioned wherein a deck is used for floatation and then one or more legs are jacked down to the seabed. The foundation of these jack-up platforms can be characterized into two categories: (1) pile supported designs and (2) gravity base structures. An example of a gravity base, jack-up tower is shown in United States Herrmann et al. Pat. No. 4,265,568. Again, although a single leg jack-up has advantages in water depths of a few hundred feet, it is still not a design suitable for deep water sites.
For deep water drilling, semi-submersible platforms have been designed, such as disclosed in United States Ray et al. Pat. No. 3,919,957. In addition, tension leg platforms have been used such as disclosed in United States Steddum Pat. No. 3,982,492. A tension leg platform includes a platform and a plurality of relatively large legs extending downwardly into the sea. Anchors are fixed to the seabed beneath each leg and a plurality of permanent mooring lines extend between the anchors and each leg. These mooring lines are tensioned to partially pull the legs, against their buoyancy, into the sea to provide stability for the platform. An example of a tension leg platform is depicted in United States Ray et al. Pat. No. 4,281,613.
In even deeper water sites, turret moored drillships and dynamically positioned drillships have been used. Turret moored drillships are featured in United States Richardson et al. Pat. Nos. 3,191,201 and 3,279,404.
A dynamically positioned drillship is similar to a turret moored vessel wherein drilling operations are conducted through a large central opening or moon pool fashioned vertically through the vessel amid ships. Bow and stern thruster sets are utilized in cooperation with multiple sensors and computer controls to dynamically maintain the vessel at a desired latitude and longitude station. A dynamically positioned drillship and riser angle positioning system is disclosed in United States Dean Pat. No. 4,317,174.
Each of the above-referenced patented inventions are of common assignment with the subject application.
Notwithstanding extensive success in shallow to medium depth drilling, there is a renewed belief that significant energy reserves exist beneath deep water of seven thousand to twelve thousand feet or more. The challenges of drilling exploratory wells to tap such reserves, however, and follow on developmental drilling over a plurality of such wells, are formidable. In this it is believed that methods and apparatus existing in the past will not be adequate to economically address the new deep water frontier.
As drilling depths double and triple, drilling efficiency must be increased and/or new techniques envisioned in order to offset the high day rates that will be necessary to operate equipment capable of addressing deep water applications. This difficulty is exacerbated for field development drilling where drilling and completion of twenty or more wells is often required. In addition, work over or remedial work such as pulling trees or tubing, acidifying the well, cementing, recompleting the well, replacing pumps, etc. in deep water can occupy a drilling rig for an extended period of time.
Accordingly, it would be desirable to provide a novel method and apparatus that would be suitable for all offshore applications but particularly suited for deep water exploration and/or developmental drilling applications that would utilize drillships, semi-submersible, tension leg platforms, and the like, with enhanced efficiency to offset inherent increases in cost attendant to deep water applications.